Electric gaseous discharge device



Feb. 23, 1937. v. J. FRANCIS 1,9 3

ELECTRIC GASEOUS DISCHARGE DEVICE Filed Oct. 12, 1955 Inventor: Victor James Francis by i/w afd H15 Attorney Patented Feb. 23, 1937 UNITED STATES 2,071,973 ELECTRIC GASEOUS DISCHABGE DEVICE Victor James Francis, Middlesex, England, assignor to General Electric Company, a corporation of New York Application October 12, 1935, Serial No. 44,795 In Great Britain October 12, 1934 6 Claims.

The presentinvention relates to electric gaseous discharge devices, and particularly to thermionic cathodes for use therein.

A particular object of the invention is to provide an electric gaseous discharge device which will have a long useful life. A further object of the invention is to provide a thermionic cathode of novel design which will have a low sputtering rate. Still other objects and advantages of the invention will appear from the following detailed specification or from an inspection of the accompanying drawing.

The invention consists in the new and novel structure hereinafter set forth and claimed.

In order to increase the luminous emciency of electric gaseous discharge devices it is now customary to provide these devices with thermionic cathodes. A cathode which is extensively used for this purpose, and especially in certain high intensity mercury vapor arc lamps, consists of a helix of refractory metal, such as tungsten, within which there is enclosed a rod-like pellet of an alkaline earth or other suitable electron emitting compound. Such a cathode is ordinarily maintained at its electron emitting temperature by the discharge. Lamps of this character have been extensively sold and have proved very successful, with a long useful life. The present invention, however, increases this already long life, and at the same time decreases the rate of depreciation, thus making these devices even more valuable to the public. In any of thesedevices the lamp gradually blackens as a-result of sputtering and evaporation of material from the cathode, and the useful life of the device is largely determined by this blackening, since a point is eventually reached at which it is no longer economical to operate the blackened device.

I have now discovered that this blackening of the envelope is greatly reduced, and the life of the device thereby greatly increased, by enclosing the pellet of active material within a housing which may comprise, for example, a plurality of grid-like members forming a series of protective layers around the pellet. In the preferred case these electrodes are formed as a plurality of concentric helices of tungsten or other suitable wire. Such a cathode is easily formed by winding a tungsten helix with free ends which are adapted to be attached to the inleads or other support, slipping the rod-like pellet of alkaline metal compound within the spiral, and then slipping a second tungsten spiral over the portion of the.

first spiral which encloses the pellet. Additional spirals may then be added as desired in the same manner.

For the successful operation of a cathode of this type it is, of course, necessary that the discharge impinging upon the cathode should reduce 5 some of the alkaline earth metal compound within the pellet and then cause this reduced metal to pass to the outer surface of the electrode where it forms and maintains a layer which promotes electron emission. I have now found that the 10 blackening of the envelope is largely due to an excess concentration of this alkaline earth metal, beyond that necessary for the desired electron emission, on the surface of the cathode, and that by reducing the rate at which this metalpasses 15 from the pellet to the outer surface by means of my novel structure this excess concentration is obviated and the blackening of the envelope thereby virtually eliminated.

In general, the pellet should be in electrical connection with each of the grid-like layers, but aside from this it is immaterial how the various elements of my novel cathode are supported. Thus the various helices or other members may be in contact or separated from each other throughout their length, although in some cases, as where these elements are to be degassed with the aid of an induction furnace, the separated arrangement facilitates the necessary heating of the various elements. 30

For the purpose of illustrating my invention I have shown two different embodiments thereof in the'accompanying drawing, in which Fig. 1 is an elevational view, in part section, of a high intensity mercury vapor arc lamp employing my novel cathodes. I

Fig. 2 is a side elevational view of one of the cathodes of Fig. 1, and

Figs. 3 and 4 are front and side views, respectively, of a modification of the cathode shown in 40 Fig. 1.

As shown in this drawing each of the cathodes has a cylindrical pellet l of a suitable alkaline earth compound, such as the mixture of silicates formed by mixing 2.5 gram molecules of barium carbonate with 1 gram molecule each of calcium carbonate and silica and firing the mixture at l600 0., although it is to be understood that various other alkaline earth compounds may likewise be used for this pellet. Immediately surrounding the pellet I and in contact therewith there is a helix ff of tungsten, while outside of the helix 2 and preferably concentric therewith there is a second tungsten helix -3. In a prefen-ed case the helix 2 is formed of wire 0.3 m. m.

in diameter, and the helix 3 of wire 0.4 m. m. in diameter. A pair of support wires 4 and 4', which also serve as leads, are connected to the inleads 6 and 5' which are sealed through the pinch 6 into the envelope I.

In the preferred embodiment of Figs. 1 and 2 the outer helix 3 is much larger than the inner helix 2 so that there is a considerable gap between the two helices. The pellet I is held in a suitable manner within the inner helix 2.. One end of each helix is welded to the support 4. The other end of the inner helix 2 is free, whilethe other end of the outer helix 3 is welded to the support 4'. With this structure it is, of course, very simple to heat the entire electrode structure by an induction furnace during the degassing process employed in the manufacture of the device. The separated helices also assist in reducing the rate at which the alkaline metaFarrives at the surface of the helix 3.

This separation of the helices is not essential,

however, to the success of my novel structure.

Thus as shown in Figs. 3 and 4 the outer helix 3 fits closely over the inner helix 2 which in turn holds the pellet I. In this case the outer helix alone is welded to the supports 4 and 4' since ample electrical connection is provided by contact between the helices 2 and 3.

It has been found that the best results are attained when my novel cathode is arranged with its axis perpendicular to the path of the discharge, as shown in Fig. 1, so that the arc plays upon the side of the helix 3, although where the ends of the helices are closed by a similar gridlike, multi-layer structure this orientation of the electrode is not so important.

The envelope 1 is filled with any suitable gas or vapor, but is especially designed to operate with unsaturated mercury vapor, as disclosed in my Patent 1,948,261, granted February 20, 1934. It is likewise preferred to enclose the envelope 7 within an outer jacket, as shown in that patent, although this is, of course, not essential to the successful operation of my novel cathode structure.

In the operation of either of the structures illustrated the discharge impinges upon the helix 3, heating it, together with the rest of the electrode assembly, whereby a copious electron emission is obtained from the. thin film of alkaline earth metal on the surface thereof. At the same time this discharge results in the reduction of more of the pellet i which is within the helix 2, and in the passage of some of the reduced metal to the surface of the helix 3, where it replaces any of the film that has been lost. Due to the tortuous path; however, between the pellet I and the outer surface of the helix 3 the rate at which the alkaline earth metal arrives at this surface is vastly reduced, with a consequent reduction in the concentration of the metal coating. It has been found that this results in a great decrease in the blackening of the envelope, without in any way decreasing the effectiveness of th'ecathode. As a result the useful life of the device is greatly increased, due both to this decrease in the rate at which the lamp blackens and to the fact that the pellet i is not used up as rapidly. Furthermore the greatly decreased rate of blackening obviously gives a higher luminous efficiency throughout the life of the device, and thereby materially enhances the value of the device.

While I have describedmy invention by reference to certain embodiments thereof it is to be understood that my invention is not limited thereto but that various omissions, substitutions and changes, within the scope of the appended claims, may be made therein without departing from the spirit of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. An electric gaseous discharge device comprising a sealed envelope containing a gaseous atmosphere, and having electrodes sealedtherein, at least one of said electrodes consisting of a thermionic cathode containing a pellet consisting at least in part of an alkaline earth compound, said, cathode having a plurality of gridlike layers of refractory metal between said pellet and the discharge path within said device.

2. An electric gaseous discharge device comprising a sealed envelope containing a gaseous atmosphere, and having electrodes sealed therein, at least one of said electrodes consisting of a thermionic cathode consisting oi. a plurality of substantially concentric helices within the innermost of which there is a pellet containing a compound of an alkaline earth metal, the axes of said helices being normal to the discharge path within said device.

3. A thermionic cathode for an electric gaseous discharge device including a containing structure which comprises a plurality of grid-like members forming a series of layers within the innermost of which is arranged a pellet consisting at least in part of a slowly vaporizable electron emitting material.

4. A thermionic cathode having a dischargereceiving outer surface and means for supplying electron emissive material to said surface; said means comprising a container structure including a plurality of concentric helices of conductive material, and a pellet consisting at least in part of a compound of an alkaline earth material arranged within the innermost of said helices.

5. A thermionic cathode comprising a container structure having an outer surface adapted to receive an electrical discharge and a supply of a slowly vaporizable electron emissive material arranged within said container structure, said structure comprising a plurality of grid-like members forming a series of separate layers efiective to retard the rate of diffusion of said electron emissive material to said outer surface.

6. A thermionic cathode structure having an outer surface thereof adapted to receive an electrical discharge and a pellet containing an alkaline earth material arranged within said structure, said cathode structure comprising a plurality of closely interfitted helices of refractory metal providing tortuous paths for the diffusion of alkaline earth material from said pellet. to said outer surface.

VICTOR JAMES FRANCIS. 

